Add NodeDiagnostics monitoring plugin

[Jim8y]

Summary

• Add the NodeDiagnostics plugin for node diagnostics telemetry, crash reporting, node liveness heartbeats, consensus stall alerts, and exception notifications.
• Capture unhandled exceptions and unobserved task exceptions, with immediate bounded flushing for fatal events.
• Report ConsensusStall error events when the local block height does not advance for ConsensusStallThresholdSeconds seconds; the default threshold is 30 seconds.
• Publish process-level heartbeat checks through Better Stack and Healthchecks-compatible URLs.
• Publish node-level liveness payloads through webhook-compatible status sinks, including block height, header height, and seconds since the local block height last advanced.
• Support configurable sinks for custom webhooks, experimental Sentry-shaped payloads, experimental Google Cloud Error Reporting payloads, Better Stack heartbeats, and Healthchecks heartbeats.
• Allow operators to configure endpoints, tokens, token headers, custom headers, heartbeat intervals, consensus stall threshold, severity thresholds, retries, request timeouts, queue limits, payload truncation limits, and operator tags.
• Enforce HTTPS for token-bearing endpoints and keep tokens out of logs.
• Add an optional startup diagnostic event so operators can validate endpoint, token, header, and payload settings during rollout.
• Add operator-facing documentation with supported providers, sample configuration, security guidance for tokens, rollout validation, provider notes, and node liveness behavior.
• Improve reliability with per-request timeouts, bounded queue drop warnings, retry controls, a User-Agent header, bounded worker shutdown, one alert per stalled height, and focused tests for timeout/configuration/payload behavior.
• Rename plugin files, directories, namespaces, configuration file, and test project from NodeOps to NodeDiagnostics.

Validation

• dotnet format --no-restore --verify-no-changes --verbosity diagnostic
• dotnet test tests/Neo.Plugins.NodeDiagnostics.Tests/Neo.Plugins.NodeDiagnostics.Tests.csproj (20 passed)
• dotnet build neo-node.sln

Note: solution build reports existing NU1903 warnings for SQLitePCLRaw.lib.e_sqlite3 in SQLiteWallet projects.

[codecov]

Codecov Report

❌ Patch coverage is 66.55405% with 297 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.16%. Comparing base (f3eb4aa) to head (f2c19c7).

Files with missing lines	Patch %	Lines
...ugins/NodeDiagnostics/NodeDiagnosticsDispatcher.cs	63.61%	105 Missing and 22 partials ⚠️
plugins/NodeDiagnostics/NodeDiagnosticsPlugin.cs	0.00%	82 Missing ⚠️
plugins/NodeDiagnostics/NodeDiagnosticsEvent.cs	77.94%	32 Missing and 13 partials ⚠️
plugins/NodeDiagnostics/NodeDiagnosticsSettings.cs	82.99%	12 Missing and 13 partials ⚠️
...ins/NodeDiagnostics/NodeDiagnosticsSinkSettings.cs	83.01%	6 Missing and 12 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           master-n3    #1078      +/-   ##
=============================================
+ Coverage      50.33%   51.16%   +0.82%     
=============================================
  Files            280      285       +5     
  Lines          16500    17388     +888     
  Branches        2137     2257     +120     
=============================================
+ Hits            8305     8896     +591     
- Misses          7632     7869     +237     
- Partials         563      623      +60     

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